Esters are most commonly prepared by the reaction of an acid and an alcohol accompanied by the elimination of water. Esters may also be formed by reaction of an alcohol with various other reactants including acid anhydrides, acid chlorides, amides, nitriles, aldehydes, and ketones. Mixtures of acids and/or alcohols may also be used as starting materials. The same may apply for the other reactants.
The reaction conditions under which esterification is effected may be varied considerably. The reaction proceeds very slowly at room temperature but quite rapidly at elevated temperatures when the mixture is at optimum reaction temperature heat is provided to promote the reaction at that temperature. Typically one of the reactants is used in stoichiometric excess in order to drive the reaction. The other reactant is then called the limiting reagent. About 99% of the limiting reagent may be converted to an ester within a few hours. The acid or anhydride is usually the limiting reagent and the alcohol is usually present in an excess.
Because the esterification of an alcohol and an organic acid or anhydride is a reversible reaction, the esterification reaction normally does not go to completion. However, conversions of over 99% may be achieved by removing at least one of the esterification products during the reaction and this removal is typically achieved by distillation. Typically water is removed. A variety of distillation techniques are known in the art to remove the produced water from the reaction zone. One method of water removal includes carrying out the reaction in a liquid medium which may form an azeotrope having a boiling point that is lower than that of either or each component of the reaction. If the reagents and the resulting ester have boiling points above 100° C. at atmospheric pressure, then the reaction temperature can be adjusted such that no liquid medium capable of forming an azeotrope with water is required. Additionally, an entrainer may be used to aid in the distillation of the water from the reaction mixture. Inert materials such as cyclohexane, hexane, benzene, toluene, or xylene may be used as an entrainer. In addition, the reactant having the lower boiling point may also be employed as the entrainer. In this case, the reactant used as the entrainer is typically charged into the reaction mixture in excess over the stoichiometric quantities required for the reaction. Esterification processes, including those employing water removal, may be conducted in a batch or continuous mode of operation. Various esterification processes are disclosed in Volume 9 of the Kirk-Othmer Encyclopaedia of Chemical Technology, Fourth Edition (1994), pp. 762-768.
A conventional batch esterification procedure includes charging all of the reactants into the reactor at the beginning of the reaction cycle. In catalytic esterification processes, the catalyst is typically added to the reaction mixture after the batch reaches a target temperature. The temperature of the reaction mixture rises until the boiling point of the reaction mixture is achieved, at which point the entrainer, and water by-product boil out of the reaction mixture. The reaction mixture may then be heated further and is vigorously stirred to promote the reaction at that temperature. Typically, the overhead vapours are condensed, the water separated from the entrainer, and the entrainer recycled to the reactor vessel. The reaction temperature, and therefore the rate of reaction, is limited by the boiling point of the reaction mixture. When the reactant with the lower boiling point (usually the alcohol) is also used as the entrainer, its concentration is gradually reduced as the reaction proceeds. Also the concentrations of the reactants decrease during the reaction, which negatively affects the reaction rate. Thus, the reaction temperature, and, therefore, the rate constant for the reaction, increases as the reaction proceeds, irrespective whether an entrainer is used or not, particularly if heat input is continued during the course of the reaction.
Conventional esterification processes may be accomplished in two reaction steps. The first reaction step generally occurs in the absence of an esterification catalyst, while the second reaction step may include the use of an esterification catalyst. In U.S. Pat. No. 5,349,075, a two step esterification process with a first uncatalyzed esterification reaction step conducted at a temperature of at least 200° C., i.e., conditions whereby the more volatile reactant is in the gaseous phase while the less volatile reactant is in the liquid phase, followed by a catalyzed second esterification reaction step at a temperature below 100° C. is proposed. The process employs a solid acid catalyst in the second reaction step. Other background references include U.S. Pat. No. 6,235,924.
In the commercial production of esters, conversions of greater than 99% are desired.
Most esterification processes are capable of converting about 99% of the limiting reagent, such as acids, anhydrides or polyols, to an ester within a few hours of reaction time; however, after about 90% of the limiting reagent is converted, the rate of reaction tends to slow down substantially. It may take half as long again to convert the remaining 4-5% of limiting reagent as it took to convert the initial 95% thereof.
The esterification reaction produces water and since the titanium or tin esterification catalysts that are typically used are water sensitive, it is necessary to minimize contact between the water produced in the reaction and the catalyst. Accordingly, it has been practice to distil off as much as possible of any water of reaction during the initial phases of the reaction prior to addition of the catalyst. However, this process requires a relatively long cycle time and also involves boiling off and recycle of reactants, particularly the alcohol. Generally, the alcohol is recycled by reflux which is typically subcooled and can therefore lower the temperature of the reaction mixture and further prolong the reaction cycle time, in particular when heat input capabilities are limited which is usually the case in an industrial setting.
Titanates are often used as esterification catalysts. The reaction is performed at elevated temperature with vigorous stirring so that thermal and mechanical energy is imparted to the reaction mixture. About 20% excess of alcohol is used to push the equilibrium of the reaction towards the formation of the ester and to act as a water entrainer. Water is formed during the reaction and is removed with boiling alcohol. The water/boiling alcohol mixture passes to a condenser and a separator where the water is separated from the alcohol which is recycled to the reactor and the water is removed. In order to maintain sufficient alcohol boil-up pressure in the reactor can be gradually reduced as the reaction progresses.
WO2008/110306 relates to the use of a particular temperature and pressure profile during a batch esterification process.
One problem associated with this esterification process is that the reaction mixture tends to foam so that the alcohol that is boiled off during the reaction carries with it some of the acid or anhydride, some of the catalyst, and can also carry some of the ester that has been produced. This reduces the yield of the reaction and also leads to product impurities. The problem is particularly acute with the esterification of monobasic acids such as benzoic acid in view of the larger amount of water that is formed. In the esterification reaction, excessive foaming can result in the reactor overhead becoming inoperative, refluxing of emulsions, and undesirable fluctuations in the reactor. Other examples of esterification reactions in which foaming occurs are the esterification of phthalic anhydride with 2-substituted alcohols like 2-ethyl hexanol and 2-propyl heptanol but also with unbranched and branched alcohols containing sodium soap contaminants particularly when the alcohol has been purified by treatment with sodium borohydride, the foaming problems are less if the alcohol has been purified by hydrofining or in which mixtures of such alcohols are used.
In a typical reaction, the acid or anhydride and the alcohol are preheated to an intermediate temperature and then the catalyst is introduced and the temperature rose to reaction temperature. Foam formation may start when the catalyst is introduced, causing enhanced reaction water formation and the temperature rose to the reaction temperature.
It has been proposed that foaming problems may be overcome by including an additional water entraining agent other than a reactant in the reaction or employing high operating pressures. However the use of such a water entraining agent introduces additional separation requirements and can also lead to impurities in the product. The use of higher pressures requires higher temperatures which does not help with the carry over problems. Thus, there remains a need for improvements in processes for the production of esters. Embodiments of the invention provide solutions to address these problems.